[BRLY-2021-006] The stack buffer overflow vulnerability leads to arbitrary code execution in UEFI DXE driver on multiple HP devices.

Summary

BINARLY efiXplorer team has discovered a stack overflow vulnerability that allows a local priviledged user to access UEFI DXE driver and execute arbitrary code.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-006
• HP PSIRT assigned CVE identifier: CVE-2021-39300
• CERT/CC assigned case number: VU#917518
• CVSS v3.1 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected HP firmwares with confirmed impact by Binarly team

Potential impact

An attacker with local privileged access can exploit this vulnerability to elevate privileges from ring 3 or ring 0 (depends on the operating system) to a DXE Runtime UEFI application and execute arbitrary code.A malicious code installed as a result of the vulnerability exploitation in DXE driver could survive across an operating system (OS) boot process and runtime or modify NVRAM area on the SPI flash storage (to gain persistence on target platform).Additionally, this vulnerability potentially could be used by threat actors to bypass OS security mechanisms (modify privileged memory or runtime variables), influence on OS boot process, and in some cases would allow an attacker to hook or modify EFI Runtime services.

Vulnerability description

The following piece of code contains a stack overflow vulnerability (firmware name: Q92_011500.bin, driver name: 0D24, code offset: 0x171F):

...
_GetCoolControlData:
  DataSize = 8i64;
  vStatus = gRT_23B0->GetVariable(
              (CHAR16 *)L"CoolControlData",
              &gVendorGuid,
              (UINT32 *)&Attributes,
              &DataSize,
              vCoolControlDataValue);
  // If the length of the CoolControlData variable is greater than 8, then at the moment vStatus = EFI_BUFFER_TOO_SMALL
  // (this means that the GetVariable call will occur a second time)
  if ( !vStatus
    || (vStatus = gRT_23B0->GetVariable(
                    (CHAR16 *)L"CoolControlData",
                    &gVendorGuid,
                    (UINT32 *)&Attributes,
                    &DataSize,
                    vCoolControlDataValue)) == 0 )
  {
    LOBYTE(v19) = vCoolControlDataValue[1];
    LOBYTE(v20) = 1;
    sub_1A78(v20, v19);
  }
  return vStatus;
}

The value of the DataSize is initialized only once (before the first call to GetVariable service).If the length of NVRAM CoolControlData variable is greater than 8, then after the second call to GetVariable service, a stack overflow may occur, followed by execution of arbitrary code.

Below is a section of the stack for the function under consideration:

-0000000000000038 vCoolControlDataValue db ?
-0000000000000037 var_37          db ?
-0000000000000036                 db ? ; undefined
-0000000000000035                 db ? ; undefined
-0000000000000034                 db ? ; undefined
-0000000000000033                 db ? ; undefined
-0000000000000032                 db ? ; undefined
-0000000000000031                 db ? ; undefined
-0000000000000030 Buffer          dq ?
-0000000000000028 var_28          dd ?
-0000000000000024                 db ? ; undefined
-0000000000000023                 db ? ; undefined
-0000000000000022                 db ? ; undefined
-0000000000000021                 db ? ; undefined
-0000000000000020                 db ? ; undefined
-000000000000001F                 db ? ; undefined
-000000000000001E                 db ? ; undefined
-000000000000001D                 db ? ; undefined
-000000000000001C                 db ? ; undefined
-000000000000001B                 db ? ; undefined
-000000000000001A                 db ? ; undefined
-0000000000000019                 db ? ; undefined
-0000000000000018 var_18          dq ?
-0000000000000010 var_10          dd ?
-000000000000000C                 db ? ; undefined
-000000000000000B                 db ? ; undefined
-000000000000000A                 db ? ; undefined
-0000000000000009                 db ? ; undefined
-0000000000000008                 db ? ; undefined
-0000000000000007                 db ? ; undefined
-0000000000000006                 db ? ; undefined
-0000000000000005                 db ? ; undefined
-0000000000000004                 db ? ; undefined
-0000000000000003                 db ? ; undefined
-0000000000000002                 db ? ; undefined
-0000000000000001                 db ? ; undefined
+0000000000000000  s              db 16 dup(?)
+0000000000000010  r              db 8 dup(?)
+0000000000000018 Interface       dq ?                    ; offset
+0000000000000020 Attributes      dq ?
+0000000000000028 DataSize        dq ?
+0000000000000030 arg_18          dq ?                    ; offset
+0000000000000038
+0000000000000038 ; end of stack variables

Thus, by assigning a 72-byte value to CoolControlData variable, an attacker can completely overwrite the return address from the current function and execute arbitrary code.

Disclosure timeline

This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.

Acknowledgements

BINARLY efiXplorer team

References

• efiXplorer:Hunting for UEFI Firmware Vulnerabilities at Scale with Automated Static Analysis